Avoid overloading std::swap around OpenVR's Json::Value

std::swap overloads without exception specification are warned by cl.exe
and clang-cl on C++17 mode, that belongs to -Wmicrosoft-exception-spec:
  ../../third_party/openvr/src/src/json/json.h(1227,13):  error: 'swap<Json::Value, void>' is missing exception specification 'noexcept' [-Werror,-Wmicrosoft-exception-spec]
  inline void swap(Json::Value& a, Json::Value& b) { a.swap(b); }

As ADL based dispatch is preferred for this case, and upstream
jsoncpp[1] already does it, this CL adopts it too.

[1]: Upstream jsoncpp commit: https://github.com/open-source-parsers/jsoncpp/commit/1d95628ba84189cd81fbabf70add5c1baf2912c5
OpenVR SDK has not rolled to this.

Change-Id: I82494ef5aca5ff0df431f94f65ae8439c41977b5
Reviewed-on: https://chromium-review.googlesource.com/1179422Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584428}

third_party/openvr/README.chromium

@@ -18,6 +18,8 @@ of JSON_FAIL_MESSAGE because "cannot use 'throw' with exceptions disabled".
 openvr.h has been modified to remove dllimport/dllexport when building a
 non-component build so this can be built as a static library.
 Redefined VRLog to avoid logspam on startup.
+json.h has been modified to convert a std::swap overload to Json::swap, so that
+we can avoid -Wmicrosoft-exception-spec warning.
 
 Copy the correct files with these commands:
 copy %openvrsdk%\README.md %chromium%\src\third_party\openvr\src

third_party/openvr/src/src/json/json.h

@@ -1218,15 +1218,11 @@ public:
   pointer operator->() const { return &deref(); }
 };
 
-} // namespace Json
-
-
-namespace std {
-/// Specialize std::swap() for Json::Value.
-template<>
-inline void swap(Json::Value& a, Json::Value& b) { a.swap(b); }
+inline void swap(Value& a, Value& b) {
+  a.swap(b);
 }
 
+}  // namespace Json
 
 #if defined(JSONCPP_DISABLE_DLL_INTERFACE_WARNING)
 #pragma warning(pop)